Received: 3 June 2019 | Revised: 12 November 2019 | Accepted: 16 December 2019 DOI: 10.1111/fwb.13477 SPECIAL ISSUE Multitrophic biodiversity patterns and environmental descriptors of sub-Arctic lakes in northern Europe Danny C. P. Lau1,2 | Kirsten S. Christoffersen3 | Jaakko Erkinaro4 | Brian Hayden5 | Jani Heino6 | Seppo Hellsten6 | Kerstin Holmgren7 | Kimmo K. Kahilainen8 | Maria Kahlert9 | Satu Maaria Karjalainen6 | Jan Karlsson2 | Laura Forsström10 | Jennifer Lento5 | Marit Mjelde11 | Jukka Ruuhijärvi12 | Steinar Sandøy13 | Ann Kristin Schartau14 | Martin-A. Svenning15 | Tobias Vrede9 | Willem Goedkoop9 1Department of Ecology and Environmental Science, Umeå University, Umeå, Sweden 2Department of Ecology and Environmental Science, Climate Impacts Research Centre, Umeå University, Abisko, Sweden 3Freshwater Biological Section, Department of Biology, University of Copenhagen, Kobenhavns, Denmark 4Natural Resources Institute Finland (Luke), Oulu, Finland 5Biology Department, Canadian Rivers Institute, University of New Brunswick, Fredericton, Canada 6Freshwater Centre, Finnish Environment Institute, Oulu, Finland 7Department of Aquatic Resources, Institute of Freshwater Research, Swedish University of Agricultural Sciences, Drottningholm, Sweden 8Department of Forestry and Wildlife Management, Inland Norway University of Applied Sciences, Koppang, Norway 9Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden 10University of Helsinki, Helsinki, Finland 11Norwegian Institute for Water Research, Oslo, Norway 12Natural Resources Institute Finland, Helsinki, Finland 13Norwegian Environment Agency, Trondheim, Norway 14Norwegian Institute for Nature Research, Oslo, Norway 15Arctic Ecology Department, Fram Center, Norwegian Institute for Nature Research, Tromsø, Norway Correspondence Danny C. P. Lau, Department of Ecology and Abstract Environmental Science, Umeå University, 1. Arctic and sub-Arctic lakes in northern Europe are increasingly threatened by cli- SE-90187 Umeå, Sweden. Email: [email protected] mate change, which can affect their biodiversity directly by shifting thermal and hydrological regimes, and indirectly by altering landscape processes and catchment Funding information Academy of Finland, Grant/Award Number: vegetation. Most previous studies of northern lake biodiversity responses to envi- 1140903 and 1268566; Norwegian Agency ronmental changes have focused on only a single organismal group. Investigations of Environment; Norwegian Institute for Nature Research at whole-lake scales that integrate different habitats and trophic levels are cur- rently rare, but highly necessary for future lake monitoring and management. 2. We analysed spatial biodiversity patterns of 74 sub-Arctic lakes in Norway, Sweden, Finland, and the Faroe Islands with monitoring data for at least three biological focal ecosystem components (FECs)—benthic diatoms, macrophytes, phytoplankton, littoral benthic macroinvertebrates, zooplankton, and fish—that covered both pelagic and benthic habitats and multiple trophic levels. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2020 The Authors. Freshwater Biology published by John Wiley & Sons Ltd. Freshwater Biology. 2020;00:1–19. wileyonlinelibrary.com/journal/fwb | 1 2 | LAU ET al. 3. We calculated the richnessrelative (i.e. taxon richness of a FEC in the lake divided by the total richness of that FEC in all 74 lakes) and the biodiversity metrics (i.e. taxon richness, inverse Simpson index (diversity), and taxon evenness) of individual FECs using presence–absence and abundance data, respectively. We then investigated whether the FEC richnessrelative and biodiversity metrics were correlated with lake abiotic and geospatial variables. We hypothesised that (1) individual FECs would be more diverse in a warmer and wetter climate (e.g. at lower latitudes and/or elevations), and in hydrobasins with greater forest cover that could enhance the supply of terrestrial organic matter and nutrients that stimulated lake productivity; and (2) patterns in FEC responses would be coupled among trophic levels. 4. Results from redundancy analyses showed that the richnessrelative of phytoplank- ton, macrophytes, and fish decreased, but those of the intermediate trophic levels (i.e. macroinvertebrates and zooplankton) increased with decreasing latitude and/ or elevation. Fish richnessrelative and diversity increased with increasing temporal variation in climate (temperature and/or precipitation), ambient nutrient concen- trations (e.g. total nitrogen) in lakes, and woody vegetation (e.g. taiga forest) cover in hydrobasins, whereas taxon richness of macroinvertebrates and zooplankton decreased with increasing temporal variation in climate. 5. The similar patterns detected for richnessrelative of fish, macrophytes, and phyto- plankton could be caused by similar responses to the environmental descriptors, and/or the beneficial effects of macrophytes as habitat structure. By creating habitat, macrophytes may increase fish diversity and production, which in turn may promote higher densities and probably more diverse assemblages of phyto- plankton through trophic cascades. Lakes with greater fish richnessrelative tended to have greater average richnessrelative among FECs, suggesting that fish are a po- tential indicator for overall lake biodiversity. 6. Overall, the biodiversity patterns observed along the environmental gradients were trophic-level specific, indicating that an integrated food-web perspective may lead to a more holistic understanding of ecosystem biodiversity in future monitoring and management of high-latitude lakes. In future, monitoring should also focus on collecting more abundance data for fish and lower trophic levels in both benthic and pelagic habitats. This may require more concentrated sampling effort on fewer lakes at smaller spatial scales, while continuing to sample lakes distributed along environmental gradients. KEYWORDS climate change, fish, freshwater, macroinvertebrates, macrophytes, monitoring baseline, phytoplankton, zooplankton 1 | INTRODUCTION environmental alterations (Heino, Virkkala, & Toivonen, 2009; Wrona et al., 2013). Future increases in temperature and precipitation Northern Europe is lake-rich (i.e. approximately 6% areal coverage; are expected to be greatest in Arctic regions (IPCC, 2014), and ex- Lehner & Döll, 2004) mainly due to its glaciation history, climate, and treme climate events are expected to become more frequent (Bates, elevational gradients. Similar to circumpolar inland waters elsewhere, Kundzewicz, Wu, & Palutikof, 2008; Christensen et al., 2001; Nilsson, the biodiversity of Arctic and sub-Arctic lakes in northern Europe Polvi, & Lind, 2015). These predicted changes will alter lake thermal is increasingly threatened by climate change and human-induced stratification patterns, ice-cover, and hydrological regimes (e.g. run-off LAU ET al. | 3 patterns) of northern lakes (Hampton et al., 2017; O'Reilly et al., 2015). higher ecosystem productivity, higher taxon richness has been ob- Large-scale climate- and land-use-induced changes in landscape bio- served in phytoplankton (Weyhenmeyer et al., 2013), macrophytes geochemical processes and catchment vegetation (Elmendorf et al., (Heino & Toivonen, 2008), zooplankton (Hessen, Faafeng, Smith, 2012; Wrona et al., 2016) will also affect nutrient and carbon transport Bakkestuen, & Walseng, 2006; Shurin et al., 2007), benthic macro- (Creed et al., 2018; Hayden et al., 2019; Larsen, Andersen, & Hessen, invertebrates (Heino, 2009; Johnson & Goedkoop, 2002), and fish 2011). For example, many Swedish lakes have experienced dramatic (Reist et al., 2006). Studies focusing on a single organismal group declines in total phosphorus concentrations since the mid-1990s, and may provide important insights into the environmental variables these declines could be attributed to the combined effects of greening, that shape assemblages of this specific group, but they disregard climate-driven changes in soil properties and terrestrial organic mat- the fact that responses of a single group are not shaped only by ter input to lakes, and catchment recovery from acidification (Huser, environmental descriptors but also by the biotic interactions with Futter, Wang, & Fölster, 2018). In contrast, increasing temperature taxa at other trophic levels (Seibold, Cadotte, MacIvor, Thorn, & and nutrient concentrations in sub-Arctic Finland are evident in lakes Müller, 2018) and their habitat (Hayden et al., 2017; Johnson & of forested areas (Hayden et al., 2019; Hayden, Myllykangas, Rolls, & Goedkoop, 2002). Evaluations of Arctic and sub-Arctic freshwater Kahilainen, 2017). Changes in chemical and physical habitat character- biodiversity responses using multiple organismal groups at differ- istics will ultimately affect the biological assemblages of these lakes ent trophic levels and habitats are rare (but see Hayden et al., 2017, and the ecosystem services they supply. 2019), but essential for our understanding of how biodiversity at Changes in thermal and hydrological regimes are recognised as the whole-ecosystem level will respond to a rapidly changing envi- the major stressors of northern Fennoscandian lakes based on a re- ronment, and how monitoring programmes for Arctic and sub-Arc- cent assessment (Lento et al., 2019). These stressors,